Methods of image formation by diffusion transfer in which silver salts, such as silver halides, are used are well known. In practical terms, these methods involve, for example, processing an imagewise exposed photosensitive silver halide emulsion layer with an aqueous alkaline solution which contains a developing agent, a silver halide solvent and a film forming agent (a viscosity increasing agent), reducing the exposed silver halide grains to silver with the developing agent and converting the unexposed silver halide grains to a transferable silver salt with the silver halide solvent, allowing the silver complex salt to diffuse and transfer by imbibition into a layer which contains a silver precipitant (an image receiving layer) which is laminated with the aforementioned emulsion layer, and reducing the silver complex with a developing agent with the assistance of a silver precipitant, thereby forming a silver image.
In practice, a film unit comprising a photosensitive element in which a photosensitive silver halide layer is provided on a support, an image receiving element in which an image receiving layer which contains a silver precipitant is provided on a support and a processing element consisting of a rupturable container which contains a viscous aqueous alkaline solution which itself contains a developing agent, a silver halide solvent and a film forming agent is used. The emulsion layer of the photosensitive element is first imagewise exposed and then the photosensitive element and the image receiving elements are laminated together in such a way that the emulsion layer and the image receiving layer face each other and then passed between a pair of rollers in such a way that the processing element which is between the elements is ruptured and the viscous aqueous alkaline solution is spread therebetween. After a prescribed period of time, the image receiving element is peeled away from the photosensitive element and a print consisting of the desired image formed in the image receiving layer is obtained.
However, no method has yet been discovered for achieving a higher transfer image forming rate with such photosensitive elements or for preventing the image which is formed from taking on a metallic luster.